The present invention relates to methods for operating fuel cells on impure fuels. In particular, it relates to methods for substantially reducing the effect of or preventing carbon monoxide poisoning of fuel cell anode electrocatalysts by introduction of a variable concentration of oxygen into the fuel stream.
Electrochemical fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst typically induces the desired electrochemical reactions at the electrodes. In addition to electrocatalyst, the electrodes may also comprise a porous electrically conductive sheet material, or xe2x80x9celectrode substratexe2x80x9d, upon which the electrocatalyst is deposited. The electrocatalyst may be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon.
Solid polymer electrolyte fuel cells employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. Separator plates, or flow field plates. for directing the reactants across one surface of each electrode substrate, are disposed on each side of the MEA. Solid polymer fuel cells operate at relatively low temperatures (circa 80xc2x0 C.) compared to other fuel cell types.
A broad range of reactants can be used in electrochemical fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
The fuel stream may contain impurities that do not contribute to, and may actually inhibit, the desired electrochemical reaction. These impurities may, for example, originate from the fuel stream supply itself, or may be generated, for example, as intermediate species during the fuel cell reactions, or may be impurities entering the fuel stream from elsewhere in the system. Some of these impurities may be chemically adsorbed or physically deposited on the surface of the anode electrocatalyst, blocking the active electrocatalyst sites and preventing these portions of the anode electrocatalyst from inducing the desired electrochemical fuel oxidation reaction. Such impurities are known as electrocatalyst xe2x80x9cpoisonsxe2x80x9d and their effect on electrochemical fuel cells is known as xe2x80x9celectrocatalyst poisoningxe2x80x9d. Electrocatalyst poisoning thus results in reduced fuel cell performance, where fuel cell performance is defined as the voltage output from the cell for a given current density. Higher performance is associated with higher voltage for a given current density or higher current for a given voltage.
In the absence of countermeasures, the adsorption or deposition of electrocatalyst poisons may be cumulative, so even minute concentrations of poisons in a fuel stream, may, over time, result in a degree of electrocatalyst poisoning which is detrimental to fuel cell performance.
Reformate streams derived from hydrocarbons or oxygenated hydrocarbons typically contain a high concentration of hydrogen fuel, but typically also contain electrocatalyst poisons such as carbon monoxide. To reduce the effects of anode electrocatalyst poisoning, it is known to pre-treat the fuel supply stream prior to directing it to the fuel cell. For example, pre-treatment methods may employ catalytic or other means to convert carbon monoxide to carbon dioxide. However, known pre-treatment means for reformate streams cannot efficiently remove 100% of the carbon monoxide. Even trace amounts less than 10 parts per million (ppm can) eventually result in electrocatalyst poisoning which causes a reduction in fuel cell performance.
Substances other than carbon monoxide are also known to poison fuel cell electrocatalysts. Poisons may also be generated by the reaction of substances in the reactant streams with the fuel cell component materials. For instance, carbon monoxide or other impurities can be generated from carbon dioxide in the presence of an electrocatalyst. This can occur when there is a relative abundance of carbon dioxide and a relatively low concentration of carbon monoxide such that equilibrium conditions favor some limited carbon monoxide formation. What constitutes a poison may depend on the nature of the fuel cell. For example, whereas methanol is the fuel in a direct methanol fuel cell, in a hydrogen fuel cell operating on a methanol reformate stream, traces of unreformed methanol can be detrimental to the electrocatalyst performance.
Conventional methods for addressing the problem of anode electrocatalyst poisoning include purging the anode with an inert gas such as nitrogen. However, such purging methods involve suspending the generation of power by the fuel cell, thus a secondary power source may be needed to provide power while the fuel cell anode is being purged.
Another approach for removing poisons from an electrocatalyst comprises introducing a xe2x80x9ccleanxe2x80x9d fuel stream containing substantially no carbon monoxide or other poisons to a poisoned fuel cell anode. Where the adsorption is reversible, an equilibrium process may result in some rejuvenation of the electrocatalyst. However, a disadvantage of this approach is that it is generally not effective against irreversibly adsorbed poisons. Furthermore, the recovery of the anode electrocatalyst by such an equilibrium process can be very slow, during which time the fuel cell is not able to operate at full capacity.
Another technique for counteract carbon monoxide electrocatalyst poisoning is to continuously introduce a low concentration of oxygen into the fuel stream upstream of the fuel cell, as disclosed in U.S. Pat. No. 4,910,099. Therein, oxygen levels from about 2% to 6% were injected into fuel streams having carbon monoxide levels from about 100 to 500 ppm. However, such an oxygen bleed into the fuel stream results in some consumption of hydrogen and hence a reduction in fuel efficiency. Further, an oxygen bleed results in undesirable localized exothermic reactions at the anode, particularly near the fuel inlet, which may damage fuel cell membranes and reduce fuel cell lifetime. Further still, since in practice oxygen bleed is typically obtained by compressing air (it is often drawn from a compressed air stream provided as a fuel cell oxidant), use of an oxygen bleed may result in an additional parasitic load on a fuel cell system. Thus, the use of an excessive amount of oxygen bleed is undesirable.
To efficiently and effectively counteract carbon monoxide poisoning, it is desirable to know the approximate concentration of carbon monoxide in the fuel stream. However, directly measuring the concentration of carbon monoxide in a fuel stream can be difficult in practice. Thus, while a sensor for directly measuring carbon monoxide is desirable in a fuel cell system, often the concentration is inferred based on the known operating conditions of the fuel cell system (for example, carbon monoxide concentrations can be determined under various operating conditions in a laboratory and can then be inferred for a fuel cell system operating under similar conditions in actual use). While many early fuel cell applications may have had relatively constant operating conditions and therefore relatively consistent levels of carbon monoxide in the fuel stream, the potential applications for fuel cells are expanding. As a result, the carbon monoxide concentration in the fuel streams now tends to vary for many reasons (for example, depending on reformer temperature, or fuel cell and/or reformer load demand conditions).
A low output voltage from one or more of the fuel cells in a fuel cell system might be used as an indicator of carbon monoxide poisoning. Preferably perhaps, a sensor cell might be incorporated in the fuel cell system for this purpose. As described in U.S. Provisional Patent Application Ser. No. 60/091,531 filed Jul. 2, 1998, by the same applicant as the present application and previously incorporated herein by reference in its entirety, a sensor cell can be incorporated whose performance is more sensitive to carbon monoxide poisoning than other fuel cells in the fuel cell system. Thus, a sensor cell can be used to indicate an abnormal or undesirable operating condition (for example, high level of CO) before it affects the performance of the other fuel cells.
U.S. patent application Ser. No. 08/998,133, filed Dec. 23, 1997, by the same applicant as the present application, discloses a fuel cell operating method in which a substantially fuel-free liquid (which may contain oxygen) is periodically introduced into the fuel stream in order to cause a fuel starved condition.
Introducing a variable concentration of oxygen into an impure fuel stream can be advantageous for the removal of fuel stream impurities. If oxygen is introduced in accordance with the impurity level in the fuel stream, fuel losses, localized heating, and parasitic losses can be reduced. Further, a method involving a periodic or intermittent introduction of oxygen has been shown to be effective to remove impurities using a smaller integrated amount of oxygen than a method involving a constant introduction of oxygen.
These advantages may be obtained in a fuel cell system which includes a fuel cell operating on a stream of impure fuel supplied to the fuel cell, and which includes a mechanism for introducing oxygen into the fuel stream for reaction with an impurity in the fuel stream within the fuel cell. The method is particularly suitable for use in systems comprising solid polymer fuel cells that operate at relatively low temperatures.
It can be advantageous to vary the concentration of oxygen introduced in accordance with a fuel cell system operating characteristic indicative of the concentration of the impurity. Suitable operating-characteristics for this purpose include the voltage of a sensor fuel cell incorporated in the system that is sensitive to a particular impurity, or the concentration of an impurity monitored or measured by an impurity sensor located somewhere in the fuel stream. Additionally, other suitable operating characteristics include the voltage of another fuel cell in the system, the voltage drop across a part of a cell (for example, the voltage differential between parts of a cell associated with a partial length of a flow field), and the temperature of a component of the fuel cell system (for example, the temperature of a reformer). The concentration of oxygen introduced into the fuel stream is desirably adjusted in response to measured or monitored variations in one or more of these operating characteristics.
In principle, it can also be advantageous to vary the amount of oxygen introduced into the fuel stream independently of the operating state of the fuel cell system. For instance, it can be advantageous to introduce oxygen into the fuel stream periodically, such as in a series of periodic pulses rather than in a continuous (steady state) manner. This technique results in a similar removal of impurities, but using a smaller total amount of oxygen. It is particularly preferred to also vary the concentration of oxygen introduced in accordance with a fuel cell system operating characteristic indicative of the concentration of the impurity.
A periodic or cyclic variation in the concentration of oxygen introduced into the fuel stream can be characterized by a waveform (for example, sinusoidal, sawtooth, square wave pulse). Further, the concentration of oxygen introduced can be varied cyclically but also in proportion to an operating characteristic of the fuel cell system indicative of impurity concentration, such as in a series of pulses whose number or amplitude vary in proportion to an impurity concentration detected.
The method is particularly suitable for substantially reducing the effect of or preventing carbon monoxide poisoning of an anode electrocatalyst, although similar benefits can be expected with regards to poisons originating from carbon dioxide or other oxidizable impurities. The method is effective when the fuel stream comprises at least up to about 1000 ppm carbon monoxide.
In the preceding, oxygen introduced may be substantially pure oxygen, in a dilute oxygen stream such as air, in an oxygen containing solution, or generated in-situ from a suitable compound such as hydrogen peroxide. In a preferred embodiment of the method, for use with fuel streams comprising carbon monoxide impurity, less than about 4% air by volume is introduced in periodic pulses. The period of the pulses may be in the range from about 1 to 50 seconds. The pulse duration or width may be less than about half the period of the pulses. In addition to such periodic pulses of air, a steady baseline concentration of air bleed (for instance less than about 0.8% by volume of the fuel stream) may be maintained in the fuel stream between pulses. In this way, some lower level of oxygen bleed is maintained at all times for baseline carbon monoxide scavenging or for purposes of carbon dioxide scavenging.
Embodiments of the methods described above can substantially reduce the effect of electrocatalyst poisoning without substantially affecting the electrical output of the fuel cells in the system. As long as a sufficient concentration of fuel is maintained in the fuel stream, temporary fuel starvation can be generally avoided. Fuel starvation occurs when the fuel stoichiometry (the ratio of the amount of fuel supplied to the amount of fuel actually consumed in the electrochemical reactions) is less than 1 and is characterized by a rise in the anode voltage in the system fuel cells, possibly resulting in fuel cell reversal in some cells (that is, where the cell voltage goes below zero).
To carry out embodiments of the above method, a mechanism is included in the fuel cell system for introducing a variable amount of oxygen into the fuel stream for reaction with an impurity in the fuel stream within the fuel cell. The oxygen introducing mechanism can comprise, for example, a flow controller for periodically introducing pulses of oxygen into the fuel stream. Further, the system can comprise one or more monitoring devices for monitoring a fuel cell operating characteristic, such as those described above, and a controller responsive to the monitoring device for adjusting the concentration of oxygen introduced into the fuel stream. The monitoring device may preferably comprise a sensor fuel cell.